The present invention relates to a method for correcting misalignment between the test needles and test points during electrical testing of printed circuit boards, in particular multiple tests, that is tests on a plurality of printed circuit boards. The invention also relates to an actuating device for correction of this misalignment.
A printed circuit board (PCB) is a base of insulating material on which electrical connections are marked, normally by a chemical subtractive process, to link the various components that are to be installed, generally by soldering, on said circuit.
These printed circuit boards are tested before installation of the components, that is to say, they are checked and analyzed to ensure that all the nets present on the board are insulated from each other and that there is electrical continuity between all points in each net.
The electrical test stage for a printed circuit board therefore comes at the end of the production cycle of the circuit. The machines used to carry out this type of test normally operate with a universal contact grid, that is, with a constant pitch (generally 100 mils; 2.54 mm), also known as a "needle bed".
Since the printed circuit board to be tested has connection points between the different components arranged in a variable manner--and therefore not referable to a constant pitch grid--in order to perform the test an interfacing device (adapter or fixture) is inserted that allows the test points on the printed circuit to be electrically connected to the measuring points of the machine (belonging to the universal grid). In order to allow a correct electrical connection between the circuit board and the machine, the circuit board must be precisely positioned on the interfacing device.
The consolidated method for ensuring positioning of the circuit board on the interfacing device is to use pins which, being installed on the interfacing device, engage in suitable mechanical service holes provided on the circuit board itself.
With this method, the precision of the positioning of the circuit board to be tested with regard to the interfacing device is affected by errors in the construction of the circuit board itself and of the interface. Errors in the construction of the printed circuit board can thus make it impossible to perform the electrical test, irrespective of the actual final functioning of the printed circuit board.
It is known that by correcting the relative position of the reference pins and the interface, some types of errors in construction of printed circuit boards can be corrected, allowing electrical testing to be performed (see for example Italian patent application No. MI96/A 000273 in the name of the same applicant).
Besides single tests, that is on only one printed circuit board, it is fairly common practice to perform multiple simultaneous tests, that is on more than one printed circuit board in the same test operation, taken as one test machine cycle. The single printed circuit boards can be physically separate from each other, or arranged on a single plate or panel of base material, which is subjected to electrical testing before the various circuits provided on it are physically separated from each other.
Each circuit board being tested can be characterized by an error which can be qualitatively and quantitatively different from those of the other circuit boards under simultaneous testing.
In these cases it is not possible to perform a single test on all the circuit boards arranged in the test area of the machine, because the results would be altered by the circuit board centering error.
A possible alternative to this could be to test the various circuit boards individually, removing the plate from the test area each time to reposition it in the correct position, according to the circuit board to be tested.
This procedure, however, seems extremely long and laborious.
An object of the invention is to eliminate this drawback, in so-called multiple tests, avoiding removal of the plate after each single test, to perform possible correction of the positioning error.
Another object of the invention is to permit such multiple tests on both sides of said circuit boards without removing the plate containing the circuit boards.
Yet another object of the invention is to provide an extremely reliable device that makes it possible to perform rapid and safe correction of the positioning error of the individual circuit boards in the various stages of the multiple test.
The method for correction of the misalignment between the test needles and the test points during multiple tests on printed circuit boards is described below.
Basically, a system of micrometic movement servoactuators is provided, which makes it possible to assign desired movements to the circuit board centering pin assembly, installed in an interface, advantageously the lower one, though maintaining the reciprocal distance between the pins unchanged.
By suitably guiding the test machine, the necessary movements can be assigned to the pin assembly to correct the position of one or more printed circuit boards being tested simultaneously in the same machine cycle, and perform the test solely on that board; the necessary movements are then assigned in succession to correct the position on the interface of the other circuit boards, but always one (or more) at a time, thus performing the test individually on each circuit board.